A&A 611, A34 (2018) https://doi.org/10.1051/0004-6361/201630134 Astronomy & © ESO 2018 Astrophysics A survey for low-mass stellar and substellar members of the Hyades open cluster Stanislav Melnikov1,2, and Jochen Eislöffel1 1 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: [email protected] 2 Ulugh Beg Astronomical Institute, Astronomical str. 33, 700052 Tashkent, Uzbekistan Received 25 November 2016 / Accepted 11 November 2017 ABSTRACT Context. Unlike young open clusters (with ages < 250 Myr), the Hyades cluster (age ∼ 600 Myr) has a clear deficit of very low-mass stars (VLM) and brown dwarfs (BD). Since this open cluster has a low stellar density and covers several tens of square degrees on the sky, extended surveys are required to improve the statistics of the VLM/BD objects in the cluster. Aims. We search for new VLM stars and BD candidates in the Hyades cluster to improve the present-day cluster mass function down to substellar masses. Methods. An imaging survey of the Hyades with a completeness limit of 21m: 5 in the R band and 20m: 5 in the I band was carried out with the 2k × 2k CCD Schmidt camera at the 2 m Alfred Jensch Telescope in Tautenburg. We performed a photometric selection of the cluster member candidates by combining results of our survey with 2MASS JHKs photometry. Results. We present a photometric and proper motion survey covering 23.4 deg2 in the Hyades cluster core region. Using optical/IR colour-magnitude diagrams, we identify 66 photometric cluster member candidates in the magnitude range 14m: 7 < I < 20m: 5. The proper motion measurements are based on several all-sky surveys with an epoch difference of 60–70 yr for the bright objects. The proper motions allowed us to discriminate the cluster members from field objects and resulted in 14 proper motion members of the Hyades. We rediscover Hy 6 as a proper motion member and classify it as a substellar object candidate (BD) based on the comparison of the observed colour-magnitude diagram with theoretical model isochrones. Conclusions. With our results, the mass function of the Hyades continues to be shallow below ∼0:15 M indicating that the Hyades have probably lost their lowest mass members by means of dynamical evolution. We conclude that the Hyades core represents the “VLM/BD desert” and that most of the substeller objects may have already left the volume of the cluster. Key words. stars: low-mass – open clusters and associations: individual: Hyades – brown dwarfs – stars: luminosity function, mass function 1. Introduction evolution the lowest mass members may escape from a cluster core (“evaporate” from the cluster) due to dynamical encoun- The accurate initial mass function (MF) of Galactic open clusters ters and mix-up with the field objects (Terlevich 1987; Kroupa allows us to build up a picture of the initial conditions of cluster 1995; de La Fuente Marcos & de La Fuente Marcos 2000). formation and to investigate their further evolution. The bright Therefore, detection of these objects will be more complicated. end of the mass function has been analysed in many detailed De La Fuente Marcos & de La Fuente Marcos(2000) suggested studies of bright clusters. In the last decades, on the other hand, that this effect can already be noticeable in the clusters with ages deep photometric surveys of open clusters were focused on the of ≥200 Myr. The first surveys of the intermediate-age clusters, faint MF end reaching out to the lowest mass members. The which covered only a small percent of the cluster core areas, nearby open clusters are very convenient targets for this goal. did not find any significant population of low-mass members, In the solar neighbourhood there are a number of open clus- similar to what was found for the Pleiades. The first studies of ters including the Pleiades, the Hyades, Praesepe (M44), and the the Coma open cluster (age ∼ 500 Myr) showed that this clus- Coma Berenices open cluster (Melotte 111). Extensive deep sur- ter has a deficit of low-mass stars in comparison to the younger veys of the Pleiades (age ∼ 120 Myr) have led to the discovery of clusters (Deluca & Weis 1981; Odenkirchen et al. 1998). The a large population of very low-mass stars (VLM) and substellar recent and deeper surveys of this cluster covered a larger field members known as brown dwarfs (BDs) (Stauffer et al. 2007; and were reaching into the substellar domain. They confirmed Lodieu et al. 2012a; Zapatero Osorio et al. 2014; Bouy et al. that this deficit seems to be intrinsic and this finding (Casewell 2015). A considerable low-mass population has also been discov- et al. 2006; Melnikov & Eislöffel 2012) supports the idea that ered in some other younger stellar clusters (α Per: Lodieu et al. the depletion is caused by dynamical evolution. Recent studies 2012b; σ Ori: Peña Ramírez et al. 2012). These surveys showed of Praesepe (∼600 Myr), another open cluster of similar age, that the initial conditions of star formation in stellar clusters were based on the analysis of all-sky surveys UKIDSS (Boudreault effective in creating low-mass members. et al. 2012) and 2MASS, PPMXL, Pan-STARRS (Wang et al. However, the proximity of a cluster also has the disad- 2014) found conflicting results. Boudreault et al.(2012) found vantage of a large extension on the sky, which renders sur- that the Praesepe MF is consistent with that of the Galactic veys for cluster membership difficult. Moreover, with cluster disk population down to 0:07 M , whereas Wang et al.(2014) Article published by EDP Sciences A34, page 1 of 14 A&A 611, A34 (2018) concluded that the cluster MF shows a deficit of members below 0:3 M . Therefore, deep wide-field surveys of intermediate- aged open clusters (with ages of 450–600 Myr) are required to improve the comparison of their mass functions with those of the younger clusters and construct a more reliable scenario of how open clusters evolve with age. The Hyades open cluster (d = 46 pc) with age ∼ 600 Myr (age = 625 Myr, Perryman et al. 1998), which has a Pleiades- like stellar population, is one of the most studied open clusters located in the solar neighbourhood. The earliest wide-field search for Hyades members covering ∼110 deg2 discovered a deficit of M-type dwarfs in this cluster (Reid 1992, 1993) with respect to the solar neighbourhood mix. From these observations Reid(1992) derived a gravitational binding radius of ∼10.5 pc and a total cluster mass of 410–480 M . Deeper imaging sur- veys, which covered only small areas to reach fainter members did not lead to a discovery of any significant population of such objects (Reid & Hawley 1999; Reid & Mahoney 2000; Gizis et al. 1999). A survey covering 10.5 deg2 by Dobbie et al.(2002) also failed to find any new low-mass members and recovered only the known stellar member RHy 29 (Reid 1993). A wide-field study of the Hyades based on the recent proper motion cata- logue PPMXL, 2MASS, and Carlsberg Meridian Catalogue 14 (CMC14)1 photometry has enabled a full census of the kine- matic cluster members down to masses of ∼0.2 M in a region up to 30 pc from the cluster centre (Röser et al. 2011). Com- bining these three surveys, Röser et al.(2011) carried out a three-dimensional analysis of the cluster population and found Fig. 1. Area of the Hyades cluster mapped by our TLS imaging sur- that the Hyades have a tidal radius of ∼9 pc with clear out- vey. The total size of the surveyed area is ∼23.4 deg2. Star symbols are ward mass segregation. Using the PPMXL and Pan-STARRS1 photometrically selected Hyades member candidates, which are listed sky surveys Goldman et al.(2013) analysed the same area of the in Table A.1. Hyades and pushed the census of its members down to 0:09 M . Based on the deepest survey over 16 deg2 of the Hyades core, Bouvier et al.(2008) reported the discovery of the first two 2. TLS photometric survey and basic CCD BDs and confirmed membership of 19 low-mass stellar mem- reduction bers. Analysing the statistically significant number of Hyades We have carried out a new wide and deep imaging survey of members found in the previous studies, Bouvier et al.(2008) the Hyades open cluster (RA = 4h26m Dec = +15◦ 00) in the RI concluded that the present-day mass function of the Hyades is bands using the 2k × 2k CCD camera in the Schmidt focus of the clearly deficient in the VLM/BD domain compared to the initial 2 m Alfred Jensch Telescope in Tautenburg. A short description MF of the Pleiades, which have a similar population structure, of the RI photometric system of the CCD camera can be found but are much younger than the Hyades. Another study based on in Melnikov & Eislöffel(2012). The Hyades are a very close the all-sky surveys UKIDSS and 2MASS (Hogan et al. 2008) and open cluster and its core area (r ∼ 2:8 pc; Perryman et al. 1998) following spectroscopic analysis (Casewell et al. 2014; Lodieu extends over a very large sky area of ∼50 deg2. Our photometric et al. 2014) added several BDs to the substellar domain of the survey obtained in October–November 2006 covers 23.4 deg2 in cluster. Nevertheless, the updated MF still shows the apparent its central area, i.e. about 47% of the cluster core (see Fig.1). deficit of the lowest mass members (Lodieu et al. 2014). The total exposure time per filter (one frame) was 600 s, and In this paper, we present the results of a new deep imaging for this exposure time the limiting magnitude of the frames was survey of the Hyades open cluster obtained with the wide- estimated to be 22:5 in the I band.